Optical head and optical disk device

ABSTRACT

An optical head comprising a light source, an objective lens, a light splitting means, a light receiving element, a tracking error signal detection means, and a spherical aberration detection means, wherein the light splitting means has six regions that are divided by a first splitting line that is substantially parallel to a longitudinal direction of an information tracks, and by second and third splitting lines perpendicular to the first splitting line. The spherical aberration detection means compares a first focal point shift amount obtained by detecting the size of a light spot formed by focusing the light fluxes created by laser light passing through two regions that are disposed between the second and third splitting lines, onto the light receiving element, and a second focal point shift amount obtained by detecting the size of a light spot formed by focusing the light fluxes created by laser light passing through the four regions, which are disposed on the outer side of the second and third splitting lines, to generate a spherical aberration error signal for detecting the spherical aberration generated at the objective lens.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Division of application Ser. No. 10/530,424, filedApr. 5, 2005, which is a U.S. National Stage of Internationalapplication no. PCT/JP2003/012864, filed Oct. 8, 2003, whichapplications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to optical heads, which are capable ofdetecting spherical aberrations, that are used in optical informationrecording and reproduction apparatuses for recording information ontooptical information recording media such as optical disks, or forreproducing information that has been recorded.

BACKGROUND ART

Generally, in order to increase the recording density of optical diskdevices, it is necessary either to shorten the wavelength of the lightused to record and reproduce optical disks serving as informationrecording media or to increase the numerical aperture (NA) of theobjective lens that focuses the light onto the optical disk.

If the numerical aperture of the objective lens is increased, then alarge spherical aberration is generated because of variations in thethickness of the substrate of the optical disk, and in the case ofmulti-layer structured optical disks, changes in the thickness of thesubstrate when switching between information recording and reproductionsurfaces. If the numerical aperture of the objective lens further isenlarged, then a large coma aberration occurs if a relative inclination(referred to below as “tilt”) is generated between the optical axis ofthe objective lens and the surface of the optical disk.

Due to these spherical and coma aberrations, light spots focused on theoptical disk deteriorate, and information recording and reproductioncapability is lost. Thus, for optical disk devices in which therecording density is high, there is a need to detect these aberrationsand correct them.

For spherical aberration detection means in conventional optical diskdevices, the spherical aberration detection means described in JP2000-171346A is known.

FIG. 20 shows a structural overview of a conventional optical head 90that is capable of detecting spherical aberration.

In FIG. 20, numeral 101 denotes a semiconductor laser, numeral 104denotes a collimator lens, numeral 105 denotes an objective lens,numeral 106 denotes an optical disk, numeral 107 denotes a hologram, andnumeral 108 denotes a photodetector.

A laser light emitted from the semiconductor laser 101 is converted tosubstantially parallel light by the collimator lens 104, and is focusedby the objective lens 105 through a substrate of the optical disk 106onto a recording and reproduction information surface. The laser lightreflected by the recording and reproduction information surface of theoptical disk 106 again passes through the substrate, passes through theobjective lens 105 and the collimator lens 104, passes through thehologram 107 and is diffracted to be incident on the signal detectingphotodetector 108.

The hologram 107 has a pattern as shown in FIG. 21.

The hologram 107 contains three regions: a first region “a” that isbounded by a straight line L that is perpendicular to the optical axis,and a first semicircle E1 that is centered on the optical axis; a secondregion “b” that is bounded by the first semicircle E1, a secondsemicircle E2 that has a radius larger than the semicircle E1 and ispositioned on the same side of the straight line L as the semicircle E1,and the straight line L; and a third region “c” that is bounded by athird semicircle E3 that is on the opposite side of the straight line Lfrom the first semicircle E1 and the second semicircle E2, and thestraight line L.

The regions “a”, “b” and “c” of the hologram 107 are configured suchthat the focal spots of the light passing through the regions “a”, “b”and “c” from the optical disk 106 side, corresponding to the regions“a”, “b” and “c” are focused separately on the photodetector 108. Thatis to say, the light passing through the three regions “a”, “b” and “c”of the hologram 107 from the optical disk 106 side is formed as focalspots in three locations on the photodetector 108.

As shown in FIG. 22A to FIG. 22C, the photodetector 108 is configured byfive light receiving regions 108 a to 108 e. The light flux from thefirst region “a” of the hologram 107, of the light fluxes of the laserlight reflected by the optical disk 106, is formed as a focal spot P1 onthe borderline of the light receiving regions 108 a and 108 b, the lightflux from the second region “b” of the hologram 107 is formed as a focalspot P2 on the borderline of the light receiving regions 108 c and 108d, and the light flux from the third region “c” is formed as a focalspot P3 in the light receiving region 108 e.

Thus, when an information signal (reproduction signal) RF recorded onthe optical disk 106 is expressed using output electric signals from thelight receiving regions 108 a to 108 e, it is given by:

Reproduction signal RF=signal obtained by the light receiving region 108a+signal obtained by the light receiving region 108 b+signal obtained bythe light receiving region 108 c+signal obtained by the light receivingregion 108 d+signal obtained by the light receiving region 108 e.

When the substrate of the optical disk 106 is suitable and there is nogeneration of spherical aberration and when the focal point is correctlyformed on the optical disk 106, that is to say, when it is focused, theshape of the focal spots P1 to P3 formed on the light receiving regions108 a to 108 e are spots of substantially the same size, as shown inFIG. 22B.

Thus, the focal spot P1 of the light flux diffracted at the hologram 107is formed such that the irradiated areas of the light receiving regions108 a and 108 b are equal. That is to say, this indicates that thevalues of the electric signal obtained from the light receiving region108 a and the electric signal obtained from the light receiving region108 b are equal. In a similar manner, the focal spot P2 is formed suchthat the irradiated areas of the light receiving regions 108 c and 108 dare equal.

Generally, if the thickness of the substrate of the optical disk 106 isnot suitable, then spherical aberration occurs in focusing opticalsystems having the above-noted configuration.

FIG. 23 shows the state of the light rays when spherical aberrationoccurs. When spherical aberration occurs, there is generation of a shiftin the focal position that depends on the distance of the light ray froman optical axis “o”. That is to say, when the light ray “b” is focusedon a surface “F”, the light ray “a”, which is further from the opticalaxis “o” than the light ray “b”, is focused in front of the surface “F”,and the light ray “c”, which is further closer to the optical axis “o”than the light ray “b”, is focused behind the surface “F”.

That is to say, by detecting the state of the focus in two regions whosedistance from the optical axis “o” is different from each other, it ispossible to know the spherical aberration situation.

If spherical aberration has occurred in the focusing optical system,then even if the system is focused, that is to say, even if thedifference in the electric signal between the light receiving region 108a and the light receiving region 108 b is 0, then the difference in theelectric signal between the light receiving region 108 c and the lightreceiving region 108 d is not 0, but takes on a positive or negativevalue. Thus, this indicates that a positive or negative sphericalaberration has occurred.

If a positive or negative spherical aberration occurs in the above-notedfocusing optical system, then assuming, for example, that a positivespherical aberration occurs, since the focal position of the focal spotP2 of the light receiving regions 108 c and 108 d, which is a light fluxof the second light ray “b”, which is a further distance from theoptical axis, is in front of the light receiving surface of thephotodetector 108, the focal spot P2 is enlarged in a half donut-shapeover the light receiving region 108 d as shown in FIG. 21A. Conversely,when a negative spherical aberration occurs, since the focal position ofthe focal spot P2 of the light receiving region 108 c and the lightreceiving region 108 d is behind the light receiving surface of thephotodetector 108, the focal spot P2 is enlarged in a half donut-shapeover the light receiving region 108 c as shown in FIG. 22C.

Consequently, a spherical aberration signal SAE, which is a signalindicating that spherical aberration has occurred in the focusingoptical system, is as given below.

Spherical aberration signal SAE=signal obtained by the light receivingregion 108 c−signal obtained by the light receiving region 108 d−K×(signal obtained by the light receiving region 108 a−signal obtained bythe light receiving region 108 b), where K is a constant.

It should be noted that the means described in JP H8-212611A is known asaberration correction means for correcting spherical aberration. Ifthere is a change in the substrate thickness of optical disks, thenliquid crystal elements are controlled to correct the aberration inaccordance with a spherical aberration detection signal.

Liquid crystal elements are elements in which a liquid crystal is sealedin a section that is sandwiched between two glass substrates. When thepart through which laser light passes is divided into a plurality ofregions and an independent voltage is applied to each region, it ispossible to change the refractive index of the corresponding parts. Itis possible to alter the phase of the wavefronts by utilizing thesechanges in the refractive index. Since the phase of the laser lightchanges sectionally when the laser light contains aberrations, theaberrations can be corrected by activating the liquid crystal elementsso as to complement the altered phases. When a voltage is applied inaccordance with the degree of aberration, it is possible to correct theaberrations with greater accuracy. If spherical aberration has occurred,then the phase of the liquid crystal elements is controlled so as tominimize wavefront aberration.

The means described in International Application PCT/JP01/05366 is knownas conventional tilt detecting means for optical disk devices.

FIG. 24 shows a structural overview of a conventional optical head 80which is capable of tilt detection.

In FIG. 24, numeral 201 denotes a semiconductor laser, numeral 202denotes a beam splitter, numeral 204 denotes a collimator lens, numeral205 denotes an objective lens, numeral 206 denotes an optical disk,numeral 207 denotes a relay lens and numeral 208 denotes aphotodetector.

The laser light emitted from the semiconductor laser 201 passes throughthe beam splitter 202, is converted to substantially parallel light bythe collimator lens 204, and passes through the substrate to be focusedon the recording and reproduction information surface of the opticaldisk 206 by the objective lens 205.

The laser light reflected by the recording and reproduction informationsurface of the optical disk 206 again passes through the substrate,passes through the objective lens 205 and the collimator lens 204, isreflected by the beam splitter 202 and is guided to the signal detectingphotodetector 208 by the relay lens 207.

The light fluxes incident on the photodetector 208 as shown in FIG. 25Aare divided into six parts and are received by the light receivingregions 208 a to 208 f. A first tracking error signal TE1 is detectedusing the signals received by the light receiving regions 208 e and 208f, and a second tracking error signal TE2 is detected using the signalsreceived by the light receiving region 208 a to the light receivingregion 208 d.

Since the tracking error signals are push pull signals, the trackingerror signals TE1 and TE2 are expressed by the following formula.

Tracking error signal TE1=signal received by the light receiving region208 e−signal received by the light receiving region 208 f.

Tracking error signal TE2=(signal received by the light receiving region208 a+signal received by the light receiving region 208 b)−(signalreceived by the light receiving region 208 c+signal received by thelight receiving region 208 d).

The inclination (tilt) of the optical disk can be detected by comparingthe phases of the first tracking error signal TE1 and the secondtracking error signal TE2.

FIG. 25B is a diagram in which the scope of the light receiving regionsis superimposed on a distribution of the light intensity of the detectedlight fluxes when the optical disk is tilted in the radial direction.Although there is an asymmetry in the intensity distribution of thelight fluxes, in accordance with the tilt of the optical disk, a largeportion of that asymmetry occurs on the light receiving regions 208 eand 208 f, as shown in FIG. 25B. Thus, the degree of influence of thetilt of the optical disk differs between the first tracking error signalTE1 and the second tracking error signal TE2.

When the optical disk is not tilted, the phase of the two tracking errorsignals TE1 and TE2 coincide, but when the optical disk tilts, a phaseshift is created between the two tracking error signals TE1 and TE2.Since the degree of influence of the tilt of the optical disk differsfor each signal, it is possible to detect the tilt of the optical diskby comparing the phases of the first tracking error signal TE1 and thesecond tracking error signal TE2.

It should be noted that even when a conventional push pull signal, thatis to say, a push pull signal TE3 obtained by the calculation, push pullsignal TE3=(signal received by the light receiving region 208 a+signalreceived by the light receiving region 208 b+signal received by thelight receiving region 208 e)−(signal received by the light receivingregion 208 c+signal received by the light receiving region 208 d+signalreceived by the light receiving region 208 f), is used as the firsttracking error signal TE1, it is possible to detect the tilt of theoptical disk. This is because, since the push pull signal TE3corresponds to a signal that is the sum of the tracking error signalsTE1 and TE2, a sufficiently detectable phase difference is generatedbetween the first and second tracking error signals TE1 and TE2.

However, in a conventional configuration, because the sphericalaberration detection means and the tilt detecting means are configuredindependently for detecting both aberrations, there is the problem ofmutual interference between the pattern of the hologram and the patternof the photodetector. Alternatively, because the number of divisions ofthe light fluxes increases, the electric signal detected by the lightreceiving regions decreases, there is the problem that the S/N rationecessary for detecting aberration cannot be obtained.

It is an object of the present invention to realize simultaneouslyspherical aberration detection and tilt detection using theconfiguration of a simple hologram and photodetector, and also toprovide an optical head of a simple configuration that includes atracking error signal detection and focus error signal detection capableof correcting offset when the objective lens is moved.

DISCLOSURE OF INVENTION

The optical head according to the present invention includes: a lightsource for radiating laser light, an objective lens for focusing thelaser light that is radiated from the light source onto an informationrecording medium, light splitting means for spatially dividing the laserlight that is reflected by the information recording medium and thatpasses through the objective lens into a plurality of light fluxes, alight receiving element for receiving the plurality of light fluxesdivided by the light splitting means, tracking error signal detectionmeans for detecting a tracking error signal based on the plurality oflight fluxes received by the photodetector, and spherical aberrationdetection means for detecting spherical aberration that occurs at theobjective lens, based on the plurality of light fluxes received by thelight receiving element. The light splitting means has six regions thatare divided by a first splitting line that is substantially parallel toa longitudinal direction of an information track formed on theinformation recording medium, and second and third splitting linesarranged in parallel that are substantially perpendicular to the firstsplitting line, and that are substantially symmetrical about the opticalaxis of the focusing optical system. The tracking error signal detectionmeans generates the tracking error signal by calculating signalsdetected by receiving light fluxes created by laser light passingthrough those two of the six regions that are disposed between thesecond and third splitting lines, and generates a signal for correctingthe offset of the tracking error signal caused by movement of theobjective lens by calculating signals detected by receiving light fluxescreated by laser light passing through those four of the six regionsthat are disposed on the outer side of the second and third splittinglines. The spherical aberration detection means compares a first focalpoint shift amount obtained by detecting the size of a light spot formedby focusing the light fluxes created by laser light passing through tworegions that are disposed between the second and third splitting linesonto the light receiving element, and a second focal point shift amountobtained by detecting the size of a light spot formed by focusing thelight fluxes created by laser light passing through the four regionsthat are disposed on the outer side of the second and third splittinglines onto the light receiving element, to generate a sphericalaberration error signal for detecting the spherical aberration generatedat the objective lens.

Another optical head according to the present invention includes: alight source for radiating laser light, an objective lens for focusingthe laser light that is radiated from the light source onto aninformation recording medium, light splitting means for spatiallydividing the laser light that is reflected by the information recordingmedium and that passes through the objective lens into a plurality oflight fluxes, a light receiving element for receiving the plurality oflight fluxes divided by the light splitting means, tracking error signaldetection means for detecting a tracking error signal, based on theplurality of light fluxes received by the light receiving element, andspherical aberration detection means for detecting spherical aberrationthat occurs at the objective lens, based on the plurality of lightfluxes received by the light receiving element. The light splittingmeans has eight regions that are divided by a first splitting line thatis substantially parallel to the longitudinal direction of theinformation track formed on the information recording medium, a secondsplitting line that is substantially perpendicular to the firstsplitting line and that passes through the optical axis of the objectivelens, a third splitting line that is substantially parallel to thesecond splitting line and a semicircle-shaped fourth splitting line thatis on the other side of the third splitting line to the second splittingline, and that is centered on the optical axis of the focusing opticalsystem. The tracking error signal detection means generates the trackingerror signal by calculating signals detected by receiving light fluxescreated by laser light passing through those two of the eight regionsthat are disposed between the second and third splitting lines, andgenerates a signal for correcting the offset of the tracking errorsignal caused by movement of the objective lens, by calculating signalsdetected by receiving light fluxes created by laser light passingthrough those two of the eight regions that are disposed on the outerside of the third splitting lines. The spherical aberration detectionmeans compares a first focal point shift amount obtained by comparingthe quantity of light detected on a first light receiving region and asecond light receiving region by focusing the light fluxes created bylaser light passing through the two regions enclosed by the secondsplitting line and the fourth splitting line, onto the splitting line ofthe first light receiving region and the second light receiving regionformed on the light receiving element, and a second focal point shiftamount obtained by comparing the quantity of light detected on a thirdlight receiving region and a fourth light receiving region by focusingthe light fluxes created by laser light passing through the two regionson the outer side of the fourth splitting line, onto the splitting lineof the third light receiving region and the fourth light receivingregion formed on the light receiving element, to generate a sphericalaberration error signal for detecting the spherical aberration generatedat the objective lens.

Yet another optical head according to the present invention includes: alight source for radiating laser light, an objective lens for focusingthe laser light that is radiated from the light source onto aninformation recording medium, light splitting means for spatiallydividing the laser light that is reflected by the information recordingmedium and that passes through the objective lens into a plurality oflight fluxes, a light receiving element for receiving the plurality oflight fluxes divided by the light splitting means, tracking error signaldetection means for detecting a tracking error signal based on theplurality of light fluxes received by the light receiving element, andspherical aberration detection means for detecting spherical aberrationthat occurs at the objective lens, based on the plurality of lightfluxes received by the light receiving element. The light splittingmeans has eight regions that are divided by a first splitting line thatis substantially parallel to the longitudinal direction of theinformation track formed on the information recording medium, second andthird splitting lines arranged in parallel that are substantiallyperpendicular to the first splitting line and are substantiallysymmetrical about the optical axis of the objective lens, and a fourthsplitting line that is circular, positioned between the second and thirdsplitting lines and centered on the optical axis of the focusing opticalsystem. The tracking error signal detection means generates the trackingerror signal by calculating signals detected by receiving light fluxescreated by laser light passing through those two of the eight regionsthat are disposed on the outer side of the fourth splitting line andbetween the second and third splitting lines, and generates a signal forcorrecting the offset of the tracking error signal caused by movement ofthe objective lens, by calculating signals detected by receiving lightfluxes created by laser light passing through four regions, of the eightregions, that are disposed on the outer side of the second and thirdsplitting lines. The spherical aberration detection means compares afirst focal point shift amount obtained by detecting changes in theshape of a light spot that is focused on the light receiving element byapplying an astigmatism to light fluxes created by laser light passingthrough a region on the inner side of the fourth splitting line, and asecond focal point shift amount obtained by detecting changes in theshape of a light spot that is focused on the light receiving element byapplying an astigmatism to light fluxes created by laser light passingthrough the entire region of the light splitting means, to generate aspherical aberration error signal for detecting the spherical aberrationgenerated at the objective lens.

An optical disk device according to the present invention includes: anoptical head according to the present invention; and a control circuitfor attaching an electrical offset to a focus error signal to create apredetermined focal point shift, and determining the constant “k” suchthat the fluctuations of the spherical aberration error signalSAE=SAE2−k×SAE1 in the range of the predetermined focal point shift arewithin a predetermined range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural overview of an optical head according toEmbodiment 1.

FIG. 2 is a diagram showing a pattern of a hologram of the optical headaccording to Embodiment 1.

FIG. 3 is a diagram showing a pattern of a photodetector of the opticalhead according to Embodiment 1.

FIG. 4A and FIG. 4B are graphs that schematically show the change in thefocus error signal with respect to the focal point shift of theobjective lens.

FIG. 5 is a graph showing the change in a spherical aberrationcorrection signal when a focus error signal is zero.

FIG. 6A and FIG. 6B are graphs illustrating a method for setting acorrection coefficient “k2”.

FIG. 7 is a flowchart showing a procedure to determine the sphericalaberration detection signal according to Embodiment 1.

FIG. 8A and FIG. 8B are structural overviews showing another sphericalaberration correction means according to Embodiment 1.

FIG. 9 is a diagram schematically showing the state of a light flux onthe hologram when tilt occurs.

FIG. 10 is a structural overview of an optical head according toEmbodiment 2.

FIG. 11 is a diagram showing a pattern of a hologram of the optical headaccording to Embodiment 2.

FIG. 12 is a diagram showing a pattern of a photodetector of the opticalhead according to Embodiment 2.

FIG. 13 is a structural overview of an optical head according toEmbodiment 3.

FIG. 14 is a diagram showing a pattern of a hologram of the optical headaccording to Embodiment 3.

FIG. 15 is a diagram showing a pattern of a photodetector of the opticalhead according to Embodiment 3.

FIG. 16 is a diagram showing a pattern of another hologram of theoptical head according to Embodiment 3.

FIG. 17 is a structural overview of an optical head according toEmbodiment 4.

FIG. 18 is a diagram showing a pattern of a polarizing hologram of theoptical head according to Embodiment 4.

FIG. 19 is a diagram showing a pattern of an integrated lightreceiving/emitting element of the optical head according to Embodiment4.

FIG. 20 is a structural overview of a conventional optical head that iscapable of detecting spherical aberration. FIG. 21 is a diagram of apattern of a hologram of a conventional optical head that is capable ofdetecting spherical aberration.

FIG. 22A to FIG. 22C are diagrams showing patterns of a photodetector ofa conventional optical head that is capable of detecting sphericalaberration.

FIG. 23 is a diagram schematically showing the state of the light rayswhen spherical aberration occurs.

FIG. 24 is a structural overview of a conventional optical head that iscapable of detecting tilt.

FIG. 25A and FIG. 25B are diagrams showing a pattern of a photodetectorof a conventional optical head that is capable of detecting tilt.

BEST MODE FOR CARRYING OUT THE INVENTION

In an optical head according to the present embodiment, sphericalaberration detection means compares a first focal point shift amountobtained by detecting the size of a light spot formed by focusing lightfluxes created by laser light passing through two regions that aredisposed between second and third splitting lines onto a photodetector,and a second focal point shift amount obtained by detecting the size ofa light spot formed by focusing light fluxes created by laser lightpassing through four regions that are disposed on the outer side of thesecond and third splitting lines onto the photodetector, to generate aspherical aberration error signal for detecting the spherical aberrationgenerated at an objective lens. Thus, spherical aberration can bedetected with a simple configuration of a hologram and thephotodetector.

It is preferable that the optical head according to the presentinvention further includes tilt detection means for detecting therelative inclination between the objective lens and the informationrecording medium, wherein the interval between the second and thirdsplitting line is narrower than the width, in the longitudinal directionof the information track of the information recording medium, of theregion in which 0-order light and ±1-order light that are diffracted atthe information track of the information recording medium aresuperimposed; and wherein the tilt detection means compares the phase ofa first tracking error signal obtained by calculating signals detectedby receiving light fluxes created by laser light passing through tworegions that are disposed between the second and third splitting lines,and of a second tracking error signal obtained by calculating signalsdetected by receiving light fluxes created by laser light passingthrough the entire region of the light splitting means, to generate atilt error signal for detecting the relative inclination between theobjective lens and the information recording medium.

In another optical head according to the present embodiment, sphericalaberration detection means compares a first focal point shift amountobtained by comparing quantity of light detected on a first lightreceiving region and a second light receiving region by focusing lightfluxes created by laser light passing through the two regions enclosedby a second splitting line and a fourth splitting line onto thesplitting line of the first light receiving region and the second lightreceiving region formed on a photodetector, and a second focal pointshift amount obtained by comparing quantity of light detected on a thirdlight receiving region and a fourth light receiving region by focusingthe light fluxes created by laser light passing through the two regionson the outer side of the fourth splitting line onto the splitting lineof the third light receiving region and the fourth light receivingregion formed on the photodetector, to generate a spherical aberrationerror signal for detecting the spherical aberration generated at anobjective lens. Thus, spherical aberration can be detected with a simpleconfiguration of a hologram and a photodetector.

It is preferable that an optical head according to the presentembodiment further includes tilt detection means for detecting therelative inclination between the objective lens and the informationrecording medium, wherein the interval between second and thirdsplitting lines is narrower than half the width, in the longitudinaldirection of the information track of the information recording medium,of the region in which 0-order light and ±1-order light that arediffracted at the information track of the information recording mediumare superimposed, and wherein the tilt detection means compares thephase of a first tracking error signal obtained by calculating signalsdetected by receiving light fluxes created by laser light passingthrough two regions that are disposed between the second and the thirdsplitting lines, and of a second tracking error signal obtained bycalculating signals detected by receiving light fluxes created by laserlight passing through the entire region of the light splitting means, togenerate a tilt error signal for detecting the relative inclinationbetween the objective lens and the information recording medium.

In yet another optical head according to the present embodiment,spherical aberration detection means compares a first focal point shiftamount obtained by detecting changes in the shape of a light spot thatis focused on the photodetector by applying an astigmatism to lightfluxes created by laser light passing through a region on the inner sideof the fourth splitting line, and a second focal point shift amountobtained by detecting changes in the shape of a light spot that isfocused on the photodetector by applying an astigmatism to light fluxescreated by laser light passing through the entire region of the lightsplitting means, to generate a spherical aberration error signal fordetecting the spherical aberration generated at the objective lens.Thus, spherical aberration can be detected with a simple configurationof a hologram and a photodetector.

It is preferable that an optical head according to the presentembodiment further includes tilt detection means for detecting therelative inclination between the objective lens and the informationrecording medium, wherein the interval between the second and thirdsplitting line is narrower than the width, in the longitudinal directionof the information track of the information recording medium, of theregion in which 0-order light and ±1-order light that are diffracted atthe information track of the information recording medium aresuperimposed; and wherein the tilt detection means compares the phase ofa first tracking error signal obtained by calculating signals detectedby receiving light fluxes created by laser light passing through tworegions that are disposed on the outer side of the fourth splitting lineand between the second and the third splitting lines, and of a secondtracking error signal obtained by calculating signals detected byreceiving light fluxes created by laser light passing through the entireregion of the light splitting means, to generate a tilt error signal fordetecting the relative inclination between the objective lens and theinformation recording medium.

It is preferable that the signal indicating the first focal point shiftamount is SAE1 and the signal indicating the second focal point shiftamount is SAE2, and the spherical aberration error signal SAE isexpressed by: SAE=SAE2−k×SAE1, (where “k” is a constant thatsubstantially satisfies k=SAE2/SAE1 when there is no sphericalaberration and when the focal point shift amount is within apredetermined range).

It is preferable that the light splitting means includes a polarizinghologram.

It is preferable that the photodetector is an integrated lightreceiving/emitting element that is configured as a single unit with thelight source.

It is preferable that the photodetector is integrated with the lightsource and the light splitting means to form an integrated opticalelement.

It is preferable that an optical head according to the presentembodiment further includes a liquid crystal element provided betweenthe objective lens and the light splitting means, and sphericalaberration correction means for correcting the spherical aberration bychanging the phase of wavefronts that pass through a liquid crystalelement due to the application of a voltage in accordance with thespherical aberration error signal created by the spherical aberrationdetection means.

Embodiments of the present invention are described below with referenceto the drawings.

EMBODIMENT 1

FIG. 1 shows a structural overview of an optical disk device 150according to Embodiment 1 of the present invention. The optical disk 150includes an optical head 100, a control circuit 11 and a memory 15.

In the optical head 100 shown in FIG. 1, numeral 1 denotes asemiconductor laser that corresponds to a light source, numeral 2denotes a polarizing beam splitter, numeral 3 denotes a ¼ wavelengthplate, numeral 4 denotes a collimator lens, numeral 5 denotes anobjective lens that corresponds to a focusing optical system, numeral 6denotes an optical disk, numeral 51 denotes a hologram that correspondsto light splitting means, numeral 61 denotes a photodetector thatcorresponds to a light receiving element, numeral 10 denotes a sphericalaberration detector, numeral 12 denotes a focus error detector, numeral13 denotes a tracking error detector and numeral 14 denotes an objectivelens actuator. Numeral 20 denotes a liquid crystal element thatcorresponds to spherical aberration correction means.

Linearly polarized laser light that is emitted from the semiconductorlaser 1 passes through the polarizing beam splitter 2, is converted tocircularly polarized light by the ¼ wavelength plate 3, is converted tosubstantially parallel light by the collimator lens 4, passes throughthe liquid crystal element 20 and passes through the substrate to befocused on the recording and reproduction information surface of theoptical disk 6 by the objective lens 5.

The laser light reflected by the recording and reproduction informationsurface of the optical disk 6 passes again through the substrate, passesthrough the objective lens 5, the liquid crystal element 20 and thecollimator lens 4, and is converted to linearly polarized light that isdifferent from that of the forward light path by the ¼ wavelength plate,after which it is reflected by the polarizing beam splitter 2. Thepolarizing beam splitter 2 has been designed such that it allows thepolarized component of the laser light on the forward light pathsubstantially to pass, and substantially to reflect the polarizedcomponent of the laser light on the return light path.

The laser light on the return light path that is reflected by thepolarizing beam splitter 2 is spatially divided into a plurality oflight fluxes and guided to the photodetector 61 by passing through andbeing diffracted by the hologram 51. The photodetector 61 is dividedinto a plurality of light receiving regions, and light that is incidenton the light receiving regions is converted to signals depending on thequantity of light, which are transmitted to the spherical aberrationdetector 10, the focus error detector 12 and the tracking error detector13.

The signals detected by the photodetector 61 is processed by the focuserror detector 12 and the tracking error detector 13, and in accordancewith a signal that has been calculated by the focus error detector 12and the tracking error detector 13 and undergone a predetermined processin the control circuit 11, the objective lens actuator 14 drives theobjective lens 5 such that the focal spot formed on the recording andreproduction information surface of the optical disk 6 by the objectivelens 5 follows the information track formed on the recording andreproduction surface of the optical disk 6.

FIG. 2 shows an example of the pattern of the hologram 51 according toEmbodiment 1. The dashed lines in the drawing indicate the fluxes oflaser light that pass through the hologram.

As shown in FIG. 2, the hologram 51 is divided into six regions by afirst splitting line LI that is substantially parallel to the directionof the information tracks of the optical disk 6 (the border of a region51 a and a region 51 b, and of a region 51 c and a region 51 d), andsecond and third splitting lines L2 and L3 that are substantiallyperpendicular to the first splitting line L1 and that are substantiallysymmetrical about the optical axis of the objective lens 5 (the borderof the region 51 a and the region 51 c, and of the region 51 b and theregion 51 d).

Thus, the hologram 51 is divided into a plurality of regions 51 a to 51d, with the border being between the parts that include a push pullcomponent and the parts that do not.

FIG. 3 shows the pattern of the photodetector 61 according toEmbodiment 1. A plurality of light receiving regions 61 a to 61 l isdisposed on the photodetector 61.

±1-order light that has passed through the region 51 a of the hologram51 (the region outside the second and third splitting lines L2 and L3)is incident on the light receiving region 61 e and the light receivingregion 61 f of the photodetector 61. ±1-order light that has passedthrough the region 51 b of the hologram 51 (the region outside thesecond and third splitting lines) is incident on the light receivingregion 61 g and the light receiving region 61 h of the photodetector 61.±1-order light that has passed through the region 51 c of the hologram51 (one of the two regions between the second and third splitting lines)is incident on the light receiving region 61 i and the light receivingregion 61 j of the photodetector 61. ±1-order light that has passedthrough the region 51 d of the hologram 51 (the other of the two regionsbetween the second and third splitting lines) is incident on the lightreceiving region 61 k and the light receiving region 61 l of thephotodetector 61.

Furthermore, 0-order light, whose light fluxes all pass through, isfocused on the quarter regions 61 a to 61 d of the photodetector 61. Itshould be noted that ±1-order light is focused on the light receivingregions on the right side, and −1-order light is focused on the lightreceiving regions on the left side.

±1-order light that passes through the regions 51 a to 51 d of thehologram 51 is focused on points that are further than the lightreceiving surface of the photodetector 61, −1-order light is focused onpoints that are closer than the light receiving surface of thephotodetector 61, and furthermore, when the optical disk 6 is at thefocal point of the objective lens 5, the grating pattern of the hologram51 is given the refracting power of a lens such that the sizes of thelight fluxes of the ±1-order light on the light receiving surface of thephotodetector 61 are substantially equal. In this manner, the sizes ofthe light fluxes of the ±1-order light on the light receiving surface ofthe photodetector 61 changes such that the sizes are all different,depending on the focal point shift of the objective lens 5.

A focus error signal FE is obtained by:

Focus error signal FE=signal obtained by the light receiving region 61e+signal obtained by the light receiving region 61 g+signal obtained bythe light receiving region 61 i+signal obtained by the light receivingregion 61 k−(signal obtained by the light receiving region 61 f+signalobtained by the light receiving region 61 h+signal obtained by the lightreceiving region 61 j+signal obtained by the light receiving region 61l).

Furthermore, a push pull signal TE1 is obtained by:

Push pull signal TE1=signal obtained by the light receiving region 61i+signal obtained by, the light receiving region 61 j−(signal obtainedby the light receiving region 61 k+signal obtained by the lightreceiving region 61 l).

Here, if the objective lens 5 moves in the direction perpendicular tothe information tracks (radial direction), an offset is generated in thepush pull signal TE1, but using a correction signal TE2 that does notinclude a push pull component,

TE2=signal obtained by the light receiving region 61 e+signal obtainedby the light receiving region 61 f−(signal obtained by the lightreceiving region 61 g+signal obtained by the light receiving region 61h), it is possible to obtain a tracking error signal TE in which theoffset caused by movement of the objective lens 5 is corrected.

Tracking error signal TE=TE1−k1×TE2, where k1 is a correctioncoefficient. The correction coefficient “k1” is determined such thatmovement of the objective lens S within a predetermined range does notgenerate an offset in the tracking error signal TE.

Next, a method for detecting spherical aberration according to thepresent embodiment is described below.

Because the regions 51 a and 51 b, and the regions 51 c and 51 d of thehologram 51 are respective different distances from the optical axis, ifspherical aberration occurs, then their focal positions differ.Consequently, by comparing a focus error signal SAE1 obtained from thelight fluxes of the region 51 a and the region 51 b of the hologram 51,and a focus error signal SAE2 obtained from the light fluxes of theregion 51 c and the region 51 d, spherical aberration detection signalsSAE1 and SAE2 that express the size and direction of the generatedspherical aberration can be obtained.

Spherical aberration detection signal SAE1=signal obtained by the lightreceiving region 61 e+signal obtained by the light receiving region 61g−(signal obtained by the light receiving region 61 f+signal obtained bythe light receiving region 61 h), and

spherical aberration detection signal SAE2=signal obtained by the lightreceiving region 61 i+signal obtained by the light receiving region 61k−(signal obtained by the light receiving region 61 j+signal obtained bythe light receiving region 61 l).

FIG. 4A is a graph schematically showing the change in the focus errorsignal FE, the focus error signal SAE1 and the focus error signal SAE2with respect to focal point shift of the objective lens 5, for a statein which there is no spherical aberration. In a state in which there isno spherical aberration, the zero crossing points of the three focuserror signals coincide.

It should be noted that as shown in FIG. 4A, usually, the amplitude andslope of the focus error signal SAE1 and those of the focus error signalSAE2 are different from each other.

FIG. 4B is a graph schematically showing the change in the focus errorsignal FE, the focus error signal SAE1 and the focus error signal SAE2with respect to focal point shift of the objective lens 5, for a statein which there is spherical aberration.

As shown in FIG. 4B, if spherical aberration has occurred, the focuserror signals shift, but there is a difference in the shift amount ofthe focus error signal FE, the focus error signal SAE1 and the focuserror signal SAE2. The difference in the shift amount of the focus errorsignal FE, the focus error signal SAE1 and the focus error signal SAE2increases in accordance with an increase in spherical aberration.

The spherical aberration correction signal SAE is obtained by thefollowing formula:SAE=SAE2−k2×SAE1,

where k2 is a correction coefficient. FIG. 5 illustrates the change inthe spherical aberration correction signal SAE when spherical aberrationoccurs, and when the focus error signal FE=0. Since the sphericalaberration correction signal SAE is a signal that depends on the sizeand direction of the spherical aberration, depending on the size anddirection of the spherical aberration correction signal detected by thespherical aberration detector 10, it is possible to alter the phase ofthe wavefront of the laser light passing through the liquid crystalelement 20 to correct spherical aberrations by adjusting the voltagethat the spherical aberration corrector 11 applies to the liquid crystalelement 20. If the liquid crystal element 20 is used, then sphericalaberration can be corrected without using a drive mechanism such as anactuator, which makes it suitable for compact optical heads.

Thus, as shown in FIG. 6A, it is preferable to set the coefficient “k2”such that in the state in which spherical aberration does not occur, theslope of the focus error signal SAE2 and that of the focus error signalSAE1×k2 are substantially equal (satisfying k2=SAE2/SAE1 in apredetermined range “d” centered on the focal point).

In this case, as shown in FIG. 6B, in the state in which sphericalaberration does not occur, even if there is focal point shift of theobjective lens 5, then (in the predetermined range “d”) the sphericalaberration correction signal SAE is substantially zero. That is to say,since the spherical aberration correction signal SAE is not influencedby the focal point shift, the accuracy of the spherical aberrationcorrection can be improved.

Here, a more specific method for determining the correction coefficient“k2” of the optical disk device that is provided with the optical headaccording to the present embodiment is described with reference to FIG.7.

First, a focus servo is turned on (step Si). Then, in accordance with asignal calculated in the focus error detector 12, the objective lensactuator 14 drives the objective lens 5 and focuses laser light onto therecording and reproduction information surface of the optical disk 6.Next, a predetermined value “A” is set to the correction coefficient“k2” (step S2). After this, with the predetermined value A, thespherical aberration detection signal SAE in the case when thecorrection coefficient “k2” is set in advance to k2=A, is obtained bycalculation ofSAE=SAE2−(k2×SAE1),and is stored in the memory as SAE(1) (step S3).

Next, an electrical offset is added to the signal calculated in thefocus error detector 12, and a predetermined amount (for example, +d/2,which is half the range “d” shown in FIG. 6A and FIG. 6B) of focal pointshift is generated (step S4). After this, the spherical aberrationdetection signal SAE at this time is stored in the memory as SAE(2)(step S5).

In a similar manner, an opposite electrical offset is added to thesignal calculated in the focus error detector 12, and a predeterminedamount (for example, −d/2) of focal point shift is generated (step S6).Then, the spherical aberration detection signal SAE at this time isstored in the memory as SAE(3) (step S7).

The spherical aberration detection signal SAE(1) calculated with nofocal point shift, the spherical aberration detection signal SAE(2)calculated with a focal point shift of +d/2 and the spherical aberrationdetection signal SAE(3) calculated with a focal point shift of −d/2,stored in the memory, are compared (step S8) and the variance of thespherical aberration detection signals SAE(1), SAE(2) and SAE(3) arejudged as to whether they are within a predetermined range or not (stepS9).

When the variance of the spherical aberration detection signals SAE(1),SAE(2) and SAE(3) is judged to be within the predetermined range (YES instep S9), the correction coefficient is set to k2=A (step S10). When thevariance of the spherical aberration detection signals SAE(1), SAE(2)and SAE(3) is judged not to be within the predetermined range (NO instep S9), the correction coefficient is set to k2=B, where the value “B”differs from the predetermined value “A” (step S11), and the procedurereturns to step S3, to obtain again the spherical aberration detectionsignals SAE(1) to SAE(3). In this case, it is preferable that the value“B” is determined in accordance with the direction and amount of thevariance of the spherical aberration detection signals SAE(1) to SAE(3).If the variance of the spherical aberration detection signals SAE(1) toSAE(3) moves into the predetermined range due to the value “B”, then itis possible to set the correction coefficient k2=B. If they do not moveinto the predetermined range, then the spherical aberration detectionsignals SAE(1) to SAE(3) are obtained with the correction coefficientk2=C, where the value C is yet another value.

By using such a procedure, it is possible to set the correctioncoefficient k2 such that the spherical aberration detection signal SAEis less affected by the influence of focal point shift.

It should be noted that for correcting spherical aberration, apart fromusing the liquid crystal element 20 shown in FIG. 1, it is possible todrive the collimator lens 4 in the optical axis direction of the laserlight to correct spherical aberration. By moving the collimator lens 4in the direction of the light source (the semiconductor laser 1), thelight emitted from the collimator lens 4 changes from parallel light todiverging light, and by moving the collimator lens 4 in the direction ofthe objective lens 5, the light emitted from the collimator lens 4changes from parallel light to converging light.

Because changing the laser light incident on the objective lens 5 toeither diverging light or converging light generates sphericalaberration, by generating spherical aberration in a direction oppositeto the spherical aberration caused by changes in the substrate thicknessof the optical disk 6, it is possible to cancel out the sphericalaberration. For example, it is possible to cancel out sphericalaberration that occurs when the substrate thickness of the optical disk6 is thicker by setting the laser light incident on the objective lens 5to be diverging light, and conversely, it is possible to cancel outspherical aberration that occurs when the substrate thickness of theoptical disk 6 is thinner by setting the laser light incident on theobjective lens 5 to be converging light.

FIG. 8A shows a structural overview of an optical head 100A of anotherembodiment, containing a mechanism to drive the collimator lens 4 in thedirection of the optical axis of the laser light. In FIG. 8A, numeral 1denotes a semiconductor laser, numeral 4 denotes a collimator lens,numeral 5 denotes an objective lens, numeral 6 denotes an optical disk,numeral 41 denotes a motor, numeral 42 denotes a shaft and numeral 43denotes a lens holder. Driving the motor 41 rotates the shaft 42, andthe collimator lens 4 and the lens holder 43 that holds the collimatorlens 4 are capable of moving in the direction of the optical axis of thelaser light.

Thus, it is possible to drive the motor 41 in response to the size anddirection of the spherical aberration correction signal SAE such thatthe spherical aberration SAE signal is zero, and by changing theposition of the collimator lens 4, which is held in the lens holder 43,in the direction of the optical axis, it is possible to correctspherical aberration.

As shown in FIG. 8B, by moving the collimator lens 4 in the direction ofthe light source (semiconductor laser 1), the light that is emitted fromthe collimator lens 4 is diverging light, and this corrects thespherical aberration caused when the substrate of the optical disk 6 isthicker.

It should be noted that means for driving the collimator lens 4 in thedirection of the optical axis of the laser light are not limited to theabove-mentioned method that uses a motor, and it goes without sayingthat any means is possible, such as an actuator driven by a magneticcircuit or a piezoelectric element.

If spherical aberration is corrected by driving the collimator lens asabove, since there is less loss of the laser light used in recording andreproduction than when the liquid crystal element 20 is used, there isthe advantage in that loss of the light utilization efficiency can besuppressed.

Next, a method to detect tilt, which is the relative inclination betweenthe objective lens 5 and the optical disk 6, is described. FIG. 9 is adiagram schematically showing the state of the light fluxes on thehologram 51 when tilt has occurred. Because the inclination of theoptical disk 6 is generated in the radial direction of the disk, that isto say, in a direction perpendicular to the information tracks, animbalance is created in the quantity of light in the push pull signal.Since a phase shift is generated in the push pull signal when such animbalance is created, if a tracking servo operation is performed usingthis push pull signal, then the zero crossing point of the trackingerror signal and the center of the information track are offset, andwhat is known as an “off-track” occurs.

Because the effect of phase shift due to tilt appears more in the regioncentered on the optical axis (region X in FIG. 9), by comparing thephase of the push pull signal of the entire light flux to that of thepush pull signal of the region X in the center of the light flux, theamount of tilt that is generated may be detected.

The push pull signal in the region X in the center of the light flux isTE1, which is expressed by:

TE1=signal obtained by the light receiving region 61 i+signal obtainedby the light receiving region 61 j−(signal obtained by the lightreceiving region 61 k+signal obtained by the light receiving region 61l). On the other hand, the push pull signal TE0 due to the entire lightflux is detected in the regions 61 a to 61 d in which 0-order light isfocused, and is expressed by:

TE0=signal obtained by the light receiving region 61 a+signal obtainedby the light receiving region 61 b−(signal obtained by the lightreceiving region 61 c+signal obtained by the light receiving region 61d). The phase difference between the push pull signal TE1 and the pushpull signal TE0 is the amount of tilt that is generated.

It should be noted that because an offset is generated between the pushpull signal TE1 and the push pull signal TE0 in accordance with themovement of the objective lens, for tilt detection, it is preferable touse signals that are corrected in accordance with a correction signalTE2 that does not include push pull components, that is to say:a signal TE=TE1−k1×TE2, anda signal TE′=TE0−k3×TE2,(k1 and k3 are correction coefficients).

It should be noted that depending on the size and direction of the tiltthat is detected, the spherical aberration corrector 11 may adjust thecurrent driving the objective lens 5 to correct the relative tiltbetween the objective lens 5 and the optical disk 6.

EMBODIMENT 2

FIG. 10 shows a structural overview of an optical disk device 250according to another embodiment of the present invention. It should benoted that regarding structures that are the same as in Embodiment 1,the same reference numbers are used, and their description below isomitted. The optical disk device 250 includes an optical head 200, acontrol circuit 11 and a memory 15. In the optical head 200 of FIG. 10,numeral 52 denotes a hologram that corresponds to light splitting meansand numeral 62 denotes a photodetector that corresponds to a lightreceiving element.

FIG. 11 shows an example of a pattern of the hologram 52 according toEmbodiment 2.

As shown in FIG. 11, the hologram 52 is divided into six regions by afirst splitting line L4 that is substantially parallel to the directionof the information tracks of the optical disk 6 (border of a region 52 aand a region 52 b, and of a region 52 c and a region 52 d), a secondsplitting line L5 that is substantially perpendicular to the firstsplitting line L4 and that passes through the optical axis of theobjective lens 5 (border of the regions 52 c and 52 d and regions 52 eand 52 f), a third splitting line L6 that is substantially parallel tothe second splitting line L5 (border of the region 52 a and the region52 c, and of the region 52 b and the region 52 d) and asemicircle-shaped fourth splitting line L7 that is on the other side ofthe third splitting line L6 relative to the second splitting line L5,and that is centered on the optical axis of the objective lens 5 (borderof the region 52 e and the region 52 f).

Thus, the hologram 52 is divided into a plurality of regions 52 a to 52f.

FIG. 12 shows a pattern of a photodetector 62 according to Embodiment 2.A plurality of regions 62 a to 62 l is disposed on the photodetector 62.

Thus, ±1-order light that passes through the region 52 a of the hologram52 (a region on the outer side of the third splitting line) is incidenton the light receiving region 62 i of the photodetector 62, ±1-orderlight that passes through the region 52 b of the hologram 52 (a regionon the outer side of the third splitting line) is incident on the lightreceiving region 62 j of the photodetector 62, ±1-order light thatpasses through the region 52 c of the hologram 52 (one of the tworegions between the second and third splitting lines L5 and L6) isincident on the light receiving region 62 k of the photodetector 62 and±1-order light that passes through the region 52 d of the hologram 52(the other of the two regions between the second and third splittinglines L5 and L6) is incident on the light receiving region 62 l of thephotodetector 62.

±1-order light that passes through the region 52 e of the hologram 52(the region between the second and fourth splitting lines L5 and L7) isincident on the light receiving regions 62 e and 62 f of thephotodetector 62, divided into two and ±1-order light that passesthrough the region 52 f of the hologram 52 (the region on the outer sideof the fourth splitting line L5) is incident on the light receivingregions 62 g and 62 h of the photodetector 62, divided into two.

Furthermore, 0-order light, of which all light fluxes pass through, isfocused on the regions 62 a to 62 d of the photodetector 62, dividedinto four.

It should be noted that the ±1-order light of the regions 52 e and 52 fof the hologram 52 is formed such that when the optical disk 6 is at thefocal point of the objective lens 5, the areas of light emitted onto thelight receiving regions 62 e and 62 f, and 62 g and 62 h are equal,respectively. That is to say that the grating pattern on the hologram 52is set such that the respective values of the electrical signal obtainedfrom the light receiving region 62 e, the electrical signal obtainedfrom the light receiving region 62 f, the electrical signal obtainedfrom the light receiving region 62 g and the electrical signal obtainedfrom the light receiving region 62 h are equal. When set in such a way,the power output from the light receiving regions 62 e, 62 f, 62 g and62 h of the photodetector 62 changes in accordance with the focal pointshift of the objective lens 5. It should be noted that FIG. 12 expressesthe manner in which the focal point shift occurs, and ±1-order light isincident only on the regions 62 e and 62 g.

The focus error signal FE is obtained by:

FE=signal obtained by the light receiving region 62 e+signal obtained bythe light receiving region 62 g−(signal obtained by the light receivingregion 62 f+signal obtained by the light receiving region 62 h).

Furthermore, the push pull signal TE1 is obtained by:

TE1=signal obtained by the light receiving region 62 k+signal obtainedby the light receiving region 62 l.

Here, if the objective lens 5 moves in the direction perpendicular tothe information tracks, an offset is generated in the push pull signalTE1, but using a correction signal TE2 that does not include a push pullcomponent, TE2=signal obtained by the light receiving region 62 i−signalobtained by the light receiving region 62 j, it is possible to obtain atracking error signal TE in which the offset caused by movement of theobjective lens 5 is corrected.

TE=TE1−k1×TE2, where k1 is a correction coefficient. The correctioncoefficient “k1” is determined such that movement of the objective lens5 within a predetermined range does not generate an offset in thetracking error signal TE.

A method for detecting spherical aberration according to the presentembodiment is described below.

Because the region 52 e and the region 52 f of the hologram 52 aredifferent respective distances from the optical axis, their focusingpositions differ when spherical aberration occurs. Consequently, bycomparing the focus error signal SAE1, obtained from light fluxespassing through the region 52 e to the focus error signal SAE2 obtainedfrom light fluxes passing through the region 52 f, it is possible toobtain a spherical aberration error signal SAE that expresses the sizeand direction of the spherical aberration that occurs.

SAE1=signal obtained by the light receiving region 62 e−signal obtainedby the light receiving region 62 f,

SAE2=signal obtained by the light receiving region 62 g−signal obtainedby the light receiving region 62 h, and

SAE=SAE2−k2×SAE1,

where k2 is a correction coefficient. As was described in Embodiment 1,it is preferable that the correction coefficient k2 is set such that theslope of the focus error signal SAE1 when in a state in which there isno spherical aberration, and the slope of the focus error signal SAE2×k2are substantially equal (the expression k2=SAE2/SAE1 is satisfied on apredetermined range “x” centered on the focal point).

It should be noted that depending on the size and direction of thespherical aberration that is detected, the spherical aberrationcorrector 11 adjusts the voltage applied to the liquid crystal element20 to correct the spherical aberration. Furthermore, as described inEmbodiment 1, it is possible to correct the spherical aberration bydriving the collimator lens 4 in the direction of the optical axis ofthe laser light.

Next, the method for detecting tilt, which is the relative inclinationof the between the objective lens 5 and the optical disk 6, isdescribed. Because the effect of phase shift due to tilt appears mainlyin regions centered around the optical axis, it is possible to determinethe amount of tilt that is generated by comparing the push pull signalof the entire light flux to the push pull signal of the central regionof the light flux.

The push pull signal of the central region of the light flux is TE1, andthis is expressed by:

TE1=signal obtained by the light receiving region 62 k−signal obtainedby the light receiving region 62 l.

On the other hand, the push pull signal TE0 of the entire light flux isdetected by the regions 62 a to 62 d in which 0-order light is focused,and is expressed by:

TE0=signal obtained by the light receiving region 62 a+signal obtainedby the light receiving region 62 b−(signal obtained by the lightreceiving region 62 c+signal obtained by the light receiving region 62d).

The phase difference between the push pull signal TE1 and the push pullsignal TE0 is the amount of tilt that is generated.

It should be noted that since offset is generated in the push pullsignal TE1 and the push pull signal TE0 in accordance with movement ofthe objective lens, for detecting tilt, it is preferable that these arecorrected by using correction signals TE2 that do not include therespective push pull components:TE=TE1−k1×TE2, andTE′=TE0−k3×TE2,

(where k1 and k3 are correction coefficients).

It should be noted that depending on the size and direction of the tiltthat is detected, the spherical aberration corrector 11 can adjust thecurrent driving the objective lens 5 to correct the relative inclinationbetween the objective lens 5 and the optical disk 6.

EMBODIMENT 3

FIG. 13 shows a structural overview of an optical disk device 350 ofanother embodiment of the present invention. It should be noted thatregarding structures that are the same as in Embodiment 1, the samereference numbers are used, and their description below is omitted. Theoptical disk device 350 includes an optical head 300, a control circuit11 and a memory 15.

In the optical head 300 shown in FIG. 13, numeral 53 denotes a hologramthat corresponds to a light splitting means, numeral 63 denotes aphotodetector that corresponds to a light receiving element, and numeral73 denotes an anamorphic lens that applies an astigmatism to lightfluxes created by laser light passing through the hologram 53.

FIG. 14 shows an example of a pattern of the hologram 53 according tothe present embodiment. As shown in FIG. 14, the hologram 53 is dividedinto seven regions by a first splitting line L81 that is substantiallyparallel to the direction of the information tracks on the optical disk6 (border of a region 53 a and a region 53 b, and of a region 53 c and aregion 53 d), second and third splitting lines L8 and L9 that aresubstantially perpendicular to the first splitting line L81 and aresubstantially symmetrical about the optical axis of the focusing opticalsystem (border of the region 53 a and the region 53 c, and of the region53 b and the region 53 d) and a circle-shaped fourth splitting line L10that is positioned between the 27 second and third splitting lines L8and L9 and centered on the optical axis of the focusing optical system(border of the regions 53 c and 53 d and a region 53 e).

Thus, the hologram 53 is divided into a plurality of regions 53 a to 53e.

FIG. 15 shows a pattern of the photodetector 63 according to the presentembodiment. The photodetector 63 is provided with a plurality of regions63 a to 63 j.

Thus, ±1-order light that passes through the region 53 a of the hologram53 (regions on the outer side of the second and third splitting lines L8and L9) is incident on the light receiving region 63 g of thephotodetector 63, ±1-order light that passes through the region 53 b ofthe hologram 53 (regions on the outer side of the second and thirdsplitting lines L8 and L9) is incident on the light receiving region 63h of the photodetector 63, ±1-order light that passes through the region53 c of the hologram 53 (one of the two regions between the second andthird splitting lines L8 and L9) is incident on the light receivingregion 63 i of the photodetector 63 and ±1-order light that passesthrough the region 53 d of the hologram 53 (the other of the two regionsbetween the second and third splitting lines L8 and L9) is incident onthe light receiving region 63 j of the photodetector 63.

Furthermore, 0-order light, of which all light fluxes pass through, isfocused on the regions 63 a to 63 d of the photodetector 63, dividedinto four. Furthermore, ±1-order light that passes through the region 53e of the hologram 53 (the region on the inner side of the fourthsplitting line L10) is focused on the light receiving regions 63 e and63 f of the photodetector 63, divided into four.

It should be noted that the anamorphic lens 73 is set with a refractingpower of a lens such that when the focal point of the objective lens 5is on the optical disk 6, the irradiated area of the light receivingregions 63 a to 63 d is equal, and is positionally adjusted. That is tosay, it is adjusted such that the values of the electrical signalsobtained from the light receiving regions 63 a to 63 d are equal. Whenset in this manner, the power of the output from the light receivingregions 63 a, 63 c, 63 b and 63 d of the photodetector 63 changes inaccordance with the focal point shift of the objective lens 5. In asimilar manner, the power of the output from the light receiving regions63 e and 63 f of the photodetector 63 changes in accordance with thefocal point shift of the objective lens 5.

The focus error signal FE is obtained by:

FE=signal obtained by the light receiving region 63 a+signal obtained bythe light receiving region 63 c−(signal obtained by the light receivingregion 63 b+signal obtained by the light receiving region 63 d).

Furthermore, the push pull signal TE1 is obtained by:

TE1=signal obtained by the light receiving region 63 a+signal obtainedby the light receiving region 63 b−(signal obtained by the lightreceiving region 63 c+signal obtained by the light receiving region 63d).

Thus, although offset is generated in the push pull signal TE1 when theobjective lens 5 moves in a direction perpendicular to the informationtracks, by using a correction signal TE2 that does not include a pushpull component, where

TE2=signal obtained by the light receiving region 63 g−signal obtainedby the light receiving region 63 h ,

it is possible to obtain a tracking error signal TE in which the offsetcaused by movement of the objective lens 5 is corrected.TE=TE1−k1×TE2

where k1 is a correction coefficient. The correction coefficient k1 isdetermined such that movement of the objective lens 5 within apredetermined range does not generate an offset in the tracking errorsignal TE.

A method for detecting spherical aberration according to the presentembodiment is described below.

Because the distance of the region 53 e of the hologram 53 from theoptical axis is small with respect to the entire light flux, the focalposition when spherical aberration occurs differs from the amount ofdefocus of the entire light flux. Consequently, by comparing the focuserror signal SAE1 obtained from the light flux that passed through theregion 53 e to the focus error signal SAE2 obtained from the entirelight flux, it is possible to obtain a spherical aberration error signalSAE that expresses the size and direction of the spherical aberrationthat occurs.

SAE1=signal obtained by the light receiving region 63 e−signal obtainedby the light receiving region 63 f,

SAE2 (=FE)=signal obtained by the light receiving region 63 a+signalobtained by the light receiving region 63 c−(signal obtained by thelight receiving region 63 b+signal obtained by the light receivingregion 63 d), andSAE=SAE2−k2×SAE1,

where k2 is a correction coefficient. As was described in Embodiment 1,it is preferable that the correction coefficient k2 is set such that theslope of the focus error signal SAE1 when in a state in which there isno spherical aberration, and the slope of the focus error signal SAE2×k2are substantially equal (the expression k2=SAE2/SAE1 is satisfied on apredetermined range×centered on the focal point).

It should be noted that depending on the size and direction of thespherical aberration that is detected, the spherical aberrationcorrector 11 adjusts the voltage applied to the liquid crystal element20 to correct the spherical aberration. Furthermore, as described inEmbodiment 1, it is possible to correct the spherical aberration bydriving the collimator lens 4 in the direction of the optical axis ofthe laser light.

Next, the method for detecting tilt, which is the relative inclinationbetween the objective lens 5 and the optical disk 6, is described. Theeffect of phase shift due to tilt appears mainly in regions centeredaround the optical axis, and it is possible to determine the amount oftilt that is generated by comparing the push pull signal of the entirelight flux to the push pull signal of the region in the center of thelight flux.

The push pull signal of the region in the center of the light flux isTE1, and this is expressed by:

TE1=signal obtained by the light receiving region 63 g−signal obtainedby the light receiving region 63 h.

On the other hand, the push pull signal TE0 of the entire light flux isdetected by the regions 63 a to 63 d on which 0-order light is focused,and is expressed by:

TE0=signal obtained by the light receiving region 63 a+signal obtainedby the light receiving region 63 b−(signal obtained by the lightreceiving region 63 c+signal obtained by the light receiving region 63d).

The phase difference between the push pull signal TE1 and the push pullsignal TE0 is the amount of tilt that is generated.

It should be noted that since offset is generated in the push pullsignal TE1 and the push pull signal TE0 in accordance with movement ofthe objective lens 5, for detecting tilt, it is preferable that theseare corrected by using correction signals TE2 that do not include a pushpull component, that is to say:TE=TE1−k1×TE2, andTE′=TE0−k3×TE2,

(where k1 and k3 are correction coefficients).

It should be noted that depending on the size and direction of the tiltthat is detected, the control circuit 11 can adjust the current drivingthe objective lens 5 to correct the relative inclination between theobjective lens 5 and the optical disk 6.

Here, FIG. 16 shows a pattern of another hologram 53A according to thepresent embodiment. The hologram 53A shown in FIG. 16 is divided intoseven regions, from a region 53 aa to a region 53 ee. The I-shapedregion shown by hatching in the diagram is not used for signaldetection.

It is obvious that the pattern of hologram 53A shown in FIG. 16 is ableto detect spherical aberration and tilt in a similar manner to thepattern of the hologram 53 shown in FIG. 14.

Depending on the pitch of the information tracks on the optical disk 6,the thickness of the substrate and the wavelength of the semiconductorlaser 1, by using the pattern of the hologram 53A shown in FIG. 16 thereare effects such as being able to reduce the offset of the trackingerror signals due to focal point shift and tilt, and advantages such asbeing able to use the region 53 ee, which is not necessary for trackingerror signal detection, to detect spherical aberration.

EMBODIMENT 4

FIG. 17 shows a structural overview of an optical disk device 450according to another embodiment of the present invention. The opticaldisk device 450 includes an optical head 400, a control circuit 11 and amemory 15.

In the optical head 400 shown in FIG. 17, numeral 30 denotes anintegrated light receiving/emitting element that integrally combines asemiconductor laser 1 that corresponds to a light source, andphotodetectors 64 that correspond to light receiving elements, numeral 3denotes a ¼ wavelength plate, numeral 4 denotes a collimator lens,numeral 5 denotes an objective lens that corresponds to a focusingoptical system, numeral 6 denotes an optical disk, numeral 54 denotes apolarizing hologram that corresponds to light splitting means, numeral10 denotes a spherical aberration detector, numeral 12 denotes a focuserror detector, numeral 13 denotes a tracking error detector, numeral 14denotes an objective lens actuator and numeral 20 denotes a liquidcrystal element that corresponds to spherical aberration correctionmeans.

Linearly polarized laser light that is emitted from the semiconductorlaser 1 in the integrated light receiving/emitting element 30 passesthrough the polarizing hologram 54 without being diffracted, isconverted to circularly polarized light by the ¼ wavelength plate 3, isconverted to substantially parallel light by the collimator lens 4,passes through the liquid crystal element 20, and is passed through thesubstrate of the optical disk 6 to focus on the recording andreproduction information surface by the objective lens 5.

The laser light reflected by the recording and reproduction informationsurface of the optical disk 6 passes again through the substrate, passesthrough the objective lens 5, the liquid crystal element 20 and thecollimator lens 4, is converted by the ¼ wavelength plate 3 to linearlypolarized light that is polarized in a direction perpendicular to thelinearly polarized light of the forward light path, after which thelaser light is spatially divided into a plurality of light fluxes bydiffraction by the polarizing hologram 54, and is guided to thephotodetectors 64.

The photodetectors 64 are divided into a plurality of light receivingregions, and the light is converted to signals depending on the quantityof light incident on the respective light receiving regions, which aretransmitted to the spherical aberration detector 10, the focus errordetector 12 and the tracking error detector 13, to detect theinformation signal and the error signal.

Thus, the polarizing hologram 54 is configured such that withoutdiffracting linearly polarized light on the forward light path, itdiffracts substantially all the linearly polarized light on the returnlight path as ±1-order light.

FIG. 18 shows an example of a pattern of the polarizing hologram 54according to the present embodiment.

As shown in FIG. 18, the polarizing hologram 54 is divided into tenregions by a first splitting line L11 that is substantially parallel tothe direction of the information tracks on the optical disk 6 (border ofa region 54 a and a region 54 b, of a region 54 c and a region 54 d, andof a region 54 e and a region 54 f), second and third splitting linesL12 and L13 that are substantially perpendicular to the first splittingline and are substantially symmetrical about the optical axis of theobjective lens 5 (border of the region 54 a and the region 54 e, and ofthe region 54 b and the region 54 f) and fourth and fifth splittinglines L14 and L15 that are substantially parallel to 32 the second andthird splitting lines and are substantially symmetrical about theoptical axis of the objective lens 5 (border of the region 54 c and theregion 54 e, and the region 54 d and of the region 54 f).

FIG. 19 shows a pattern of the light receiving region of the integratedlight receiving/emitting element 30 according to the present embodiment.The integrated light receiving/emitting element 30 is provided with aplurality of light receiving regions 64 e to 64 n. It should be notedthat the center of the integrated light receiving/emitting element 30 isthe semiconductor laser 1.

Thus, ±1-order light that passes through the region 54 a of the hologram54 (regions on the outer side of the second and third splitting linesL12 and L13) is incident on the light receiving region 64 e and thelight receiving region 64 f of the photodetector 64, ±1-order light thatpasses through the region 54 b of the hologram 54 (regions on the outerside of the second and third splitting lines L12 and L13) is incident onthe light receiving region 64 g and the light receiving region 64 h ofthe photodetector 64, ±1-order light that passes through the region 54 cof the hologram 54 (one of the two regions between the fourth and fifthsplitting lines L14 and L15) is incident on the light receiving region64 i and the light receiving region 64 j of the photodetector 64, and±1-order light that passes through the region 54 d of the hologram 64(one of the two regions between the fourth and fifth splitting lines L14and L15) is incident on the light receiving region 64 k and the lightreceiving region 64 l of the photodetector 64.

±1-order light that passes through the region 54 e of the hologram 54(one of the regions between the second and third splitting lines L12 andL13, and on the outer side of the fourth and fifth splitting lines L14and L15) is incident on the light receiving region 64 m of thephotodetector 64, and ±1-order light that passes through the region 54 fof the hologram 54 (one of the regions between the second and thirdsplitting lines L12 and L13, and on the outer side of the fourth andfifth splitting lines L14 and L15) is incident on the light receivingregion 64 n of the photodetector 64.

It should be noted that ±1-order light at the regions 54 a to 54 d ofthe hologram 54 has a focal point that is further than the lightreceiving surface of the photodetector 64, the −1-order light at theregions 54 a to 54 d of the hologram 54 has a focal point that is closerthan the light receiving surface of the photodetector 64, and also thegrating pattern of the polarizing hologram 54 is given the refractingpower of a lens such that when the optical disk 6 is at the focal pointof the objective lens 5, the size of the light fluxes of the ±1-orderlight on the light receiving surface of the photodetector 64 issubstantially the same. By setting the polarizing hologram 54 in such amanner, the sizes of the light fluxes of the ±1-order light on the lightreceiving surface of the photodetector 64 change in accordance with thefocal point shift of the objective lens 5.

The focus error signal FE is obtained by:

FE=signal obtained by the light receiving region 64 e+signal obtained bythe light receiving region 64 g+signal obtained by the light receivingregion 64 i+signal obtained by the light receiving region 64 k−(signalobtained by the light receiving region 64 f+signal obtained by the lightreceiving region 64 h+signal obtained by the light receiving region 64j+signal obtained by the light receiving region 64 l).

Furthermore, the push pull signal TE1 is obtained by:

TE1=signal obtained by the light receiving region 64 i+signal obtainedby the light receiving region 64 j−(signal obtained by the lightreceiving region 64 k+signal obtained by the light receiving region 64l).

Thus, although offset is generated in the push pull signal TE1 when theobjective lens 5 moves in a direction perpendicular to the informationtracks, by using a correction signal TE2 that does not include a pushpull component, that is to say:

TE2=signal obtained by the light receiving region 64 e+signal obtainedby the light receiving region 64 f−(signal obtained by the lightreceiving region 64 g+signal obtained by the light receiving region 64h),

it is possible to obtain a tracking error signal TE in which the offsetcreated by movement of the objective lens 5 is corrected.TE=TE1−k1×TE2

where k1 is a correction coefficient. The correction coefficient k1 isdetermined such that movement of the objective lens 5 within apredetermined range does not generate an offset in the tracking errorsignal TE.

A method for detecting spherical aberration according to the presentembodiment is described below.

The regions 54 a and 54 b, and the regions 54 c and 54 d of thepolarizing hologram 54 are different distances from the optical axis toeach other, and thus their focal positions differ when sphericalaberration occurs. Consequently, by comparing the focus error signalSAE1 obtained from light fluxes passing through the regions 54 a and 54b of the hologram 54 to the focus error signal SAE2 obtained from lightfluxes passing through the regions 54 c and 54 d, it is possible toobtain a spherical aberration error signal SAE that expresses the sizeand direction of the spherical aberration that occurs.

SAE1=signal obtained by the light receiving region 64 e+signal obtainedby the light receiving region 64 g−(signal obtained by the lightreceiving region 64 f+signal obtained by the light receiving region 64h),

SAE2=signal obtained by the light receiving region 64 i+signal obtainedby the light receiving region 64 k−(signal obtained by the lightreceiving region 64 j+signal obtained by the light receiving region 64l), andSAE=SAE2−k2×SAE1,

where k2 is a correction coefficient. As was described in Embodiment 1,it is preferable that the correction coefficient k2 is set such that theslope of the focus error signal SAE1 when in a state in which there isno spherical aberration, and the slope of the focus error signal SAE2×k2are substantially equal (the expression k2=SAE2/SAE1 is satisfied on apredetermined range×centered on the focal point).

It should be noted that depending on the size and direction of thespherical aberration that is detected, the spherical aberrationcorrector 11 adjusts the voltage applied to the liquid crystal element20 to correct the spherical aberration. Furthermore, as described inEmbodiment 1, it is possible to correct the spherical aberration bydriving the collimator lens 4 in the direction of the optical axis ofthe laser light.

Next, a method for detecting tilt, which is the relative inclinationbetween the objective lens 5 and the optical disk 6, is described.Because the effect of phase shift due to tilt appears mainly in regionscentered around the optical axis, it is possible to determine the amountof tilt that is generated by comparing the two push pull signals, whichare mutually different distances from the optical axis.

The push pull signal close to the optical axis is the TE1, and this isexpressed by:

TE1=signal obtained by the light receiving region 64 i+signal obtainedby the light receiving region 64 j−(signal obtained by the lightreceiving region 64 k+signal obtained by the light receiving region 64l).

On the other hand, the push pull signal TE0 that is far from the opticalaxis is expressed by:

TE0=signal obtained by the light receiving region 64 m−signal obtainedby the light receiving region 64 n.

The phase difference between the push pull signal TE1 and the push pullsignal TE0 is the amount of tilt that is generated.

It should be noted that depending on the size and direction of the tiltthat is detected, the spherical aberration corrector 11 can adjust thecurrent driving the objective lens 5 to correct the relative inclinationbetween the objective lens 5 and the optical disk 6.

Although the configuration of the optical head using the integratedlight receiving/emitting element and the polarizing hologram describedin the present embodiment detects signals in a similar manner to thatbased on SSD focus detection (Embodiment 1), an optical head using suchan integrated light receiving/emitting element and a polarizing hologramis not limited to the patterns of the polarizing hologram and lightreceiving regions according to the present invention. Even if it isconfigured according to a signal detection method similar to one basedon knife edge focus detection (Embodiment 2) or based on astigmaticfocus detection (Embodiment 3), for example, the configuration of theoptics in the forward and return light paths is simple, and an opticalhead that has cost and size advantages may be obtained.

Moreover, by integrally configuring the optical system that includes theintegrated light receiving/emitting element 30, the polarizing hologram54 and the ¼ wavelength plate 3 in FIG. 17 as an integrated opticalelement, the optical head enjoys further cost and size advantages and isthus more preferable.

On the other hand, the method for correcting tilt is not limited to themethod described in the present embodiment in which the relativeinclination between the objective lens and the optical disk is correctedby driving the objective lens, and it is clear that various correctionmethods, such as activating a liquid crystal element that is capable ofphase control to cancel out only coma aberration that is generated, alsomay be applied.

INDUSTRIAL APPLICABILITY

With the optical head of the present invention, it is possible to obtainthe superior effect of adding a spherical aberration detection functionto an optical head that is provided with tracking error signal detectioncapable of correcting offset when the objective lens moves, and withwhat is known as SSD focus detection that detects the amount of focusshift from the size of a light spot, without greatly changing theconfiguration of the optical head.

Furthermore, with the optical head of the present invention, it ispossible to obtain the superior effect of adding a spherical aberrationdetection function and a tilt detection function to an optical head thatis provided with tracking error signal detection capable of correctingoffset when the objective lens moves, and with what is known as SSDfocus detection that detects the amount of focus shift from the size ofa light spot, without greatly changing the configuration of the opticalhead.

With the optical head of the present invention, it is also possible toobtain the superior effect of adding a spherical aberration detectionfunction to an optical head that is provided with tracking error signaldetection capable of correcting offset when the objective lens moves,and with what is known as knife edge focus detection that detects theamount of focal point shift by comparing the quantity of light detectedin light receiving regions that are divided into two, without greatlychanging the configuration of the optical head.

With the optical head of the present invention, it is also possible toobtain the superior effect of adding a spherical aberration detectionfunction and a tilt detection function to an optical head that isprovided with tracking error signal detection capable of correctingoffset when the objective lens moves, and with what is known as knifeedge focus detection that detects the amount of focal point shift bycomparing the quantity of light detected in light receiving regions thatare divided into two, without greatly changing the configuration of theoptical head.

With the optical head of the present invention, it is also possible toobtain the superior effect of adding a spherical aberration detectionfunction to an optical head that is provided with tracking error signaldetection capable of correcting offset when the objective lens moves,and with what is known as astigmatic focus detection that detects theamount of focal point shift from changes in the shape of a light spot towhich astigmatism is applied, on a light receiving element, withoutgreatly changing the configuration of the optical head.

With the optical head of the present invention, it is also possible toobtain the superior effect of adding a spherical aberration detectionfunction and a tilt detection function to an optical head that isprovided with tracking error signal detection capable of correctingoffset when the objective lens moves, and with what is known asastigmatic focus detection that detects the amount of focal point shiftfrom changes in the shape of a light spot to which astigmatism isapplied, on a light receiving element, without greatly changing theconfiguration of the optical head.

Moreover, with the optical head of the present invention, in the statein which spherical aberration does not occur, since the sphericalaberration correction signal is substantially zero even when focal pointshift occurs, the spherical aberration correction signal SAE is notaffected by the focal point shift. Thus, it is possible to increase theaccuracy of spherical aberration correction.

With the optical head of the present invention, it is also possible toobtain the superior effect of greatly increasing the utilizationefficiency of the laser light that is incident on the optical disk byconfiguring the optical head such that linearly polarized light emittedfrom the light source that is incident on the polarizing hologram is notdiffracted, and that linearly polarized light of the return light path,which is reflected by the optical disk and is incident on the polarizinghologram, substantially is diffracted as ±1-order light.

With the optical head of the present invention, it is also possible toobtain the superior effects of not just simplifying adjustment bydisposing the light source and the light receiving element in about thesame plane, but also advantages of miniaturization of the optical headas well as cost.

With the optical head of the present invention, it is also possible toobtain the superior effects of not just simplifying adjustment bydisposing the light source and the light receiving element in about thesame plane as well as integrally configuring the light splitting meanswhose position is set, but also advantages of miniaturization of theoptical head as well as cost.

With the optical head of the present invention, which is provided withspherical aberration correction means for correcting sphericalaberration generated by the focusing optical system, it is also possibleto obtain the superior effects of obtaining an optical head that issmall, and that is capable of detecting and correcting sphericalaberration, without the need to substantially change the configurationof a conventional optical head, because the spherical aberrationcorrection means corrects the spherical aberration by changing the phaseof wavefronts that pass through a liquid crystal element due to theapplication of a voltage in accordance with the spherical aberrationerror signal and driving mechanisms such as actuators are not used.

With the optical head of the present invention, which is provided with acollimator lens that converts laser light emitted from the light sourceto substantially parallel light, it is also possible to obtain thesuperior effect of obtaining an optical head that is capable ofdetecting and correcting spherical aberration, by just driving thecollimator lens of a conventional optical head, without substantiallychanging the optical configuration and also without reducing the lightutilization efficiency of the laser light, because the sphericalaberration generated by the focusing optical system is corrected bydisplacing the collimator lens in the direction of the optical axis ofthe focusing optical system in accordance with the spherical aberrationerror signal.

1-12. (canceled)
 13. An optical head, comprising: a light source forradiating laser light an objective lens for focusing the laser lightthat is radiated from the light source onto an information recordingmedium; light splitting device for spatially dividing the laser lightthat is reflected by the information recording medium and that passesthrough the objective lens into a plurality of light fluxes; a lightreceiving element for receiving the plurality of light fluxes divided bythe light splitting device; tracking error signal detection part fordetecting a tracking error signal based on the plurality of light fluxesreceived by the light receiving element; and spherical aberrationdetection part for detecting spherical aberration that occurs betweenthe radiation of the laser light from the light source and the focusingof the laser light onto the information recording medium, based on theplurality of light fluxes received by the light receiving element;wherein the light splitting device has eight regions that are divided bya first splitting line that is substantially parallel to thelongitudinal direction of an information track formed on the informationrecording medium, a second splitting line that is substantiallyperpendicular to the first splitting line and that passes through theoptical axis of the objective lens, a third splitting line that issubstantially parallel to the second splitting line and asemicircle-shaped fourth splitting line that is on the other side of thethird splitting line to the second splitting line, and that is centeredon the optical axis of the objective lens; wherein the tracking errorsignal detection part generates a first push pull signal by calculatingsignals detected by receiving light fluxes created by laser lightpassing through those two of the eight regions that are disposed betweenthe second and third splitting lines, and generates a signal forcorrecting the offset of the first push pull signal caused by movementof the objective lens, by calculating signals detected by receivinglight fluxes created by laser light passing through those two of theeight regions that are disposed on the outer side of the third splittinglines; and wherein the spherical aberration detection part compares afirst focal point shift amount obtained by comparing the quantity oflight detected on a first light receiving region and a second lightreceiving region by focusing the light fluxes created by laser lightpassing through the two regions enclosed by the second splitting lineand the fourth splitting line, onto the splitting line of the firstlight receiving region and the second light receiving region formed onthe light receiving element, and a second focal point shift amountobtained by comparing the quantity of light detected on a third lightreceiving region and a fourth light receiving region by focusing thelight fluxes created by laser light passing through the two regions onthe outer side of the fourth splitting line, onto the splitting line ofthe third light receiving region and the fourth light receiving regionformed on the light receiving element, to generate a sphericalaberration error signal for detecting the spherical aberration generatedbetween the radiation of the laser light from the light source and thefocusing of the laser light onto the information recording medium. 14.The optical head according to claim 13, further comprising tiltdetection part for detecting the relative inclination between theobjective lens and the information recording medium, wherein theinterval between the second and third splitting lines is narrower thanhalf the width, in the longitudinal direction of the information trackof the information recording medium, of the region in which 0-orderlight and ±1-order light that are diffracted at the information track ofthe information recording medium are superimposed; and wherein the tiltdetection part compares the phase of a first push pull signal obtainedby calculating signals detected by receiving light fluxes created bylaser light passing through two regions that are disposed between thesecond and third splitting lines, and of a second push pull signalobtained by calculating signals detected by receiving light fluxescreated by laser light passing through the entire region of the lightsplitting device, to generate a tilt error signal for detecting therelative inclination between the objective lens and the informationrecording medium.
 15. An optical head, comprising: a light source forradiating laser light an objective lens for focusing the laser lightthat is radiated from the light source onto an information recordingmedium; light splitting device for spatially dividing the laser lightthat is reflected by the information recording medium and that passesthrough the objective lens into a plurality of light fluxes; a lightreceiving element for receiving the plurality of light fluxes divided bythe light splitting device; tracking error signal detection part fordetecting a tracking error signal based on the plurality of light fluxesreceived by the light receiving element; and spherical aberrationdetection part for detecting spherical aberration that occurs betweenthe radiation of the laser light from the light source and the focusingof the laser light onto the information recording medium, based on theplurality of light fluxes received by the light receiving element;wherein the light splitting device has eight regions that are divided bya first splitting line that is substantially parallel to thelongitudinal direction of the information track formed on theinformation recording medium, second and third splitting lines arrangedin parallel that are substantially perpendicular to the first splittingline and are substantially symmetrical about the optical axis of theobjective lens, and a fourth splitting line that is circular, positionedbetween the second and third splitting lines and centered on the opticalaxis of the objective lens; wherein the tracking error signal detectionpart generates a first push pull signal by calculating signals detectedby receiving light fluxes created by laser light passing through thosetwo of the eight regions that are disposed on the outer side of thefourth splitting line and between the second and third splitting lines,and generates a signal for correcting the offset of the first push pullsignal caused by movement of the objective lens, by calculating signalsdetected by receiving light fluxes created by laser light passingthrough four regions, of the eight regions, that are disposed on theouter side of the second and third splitting lines; and wherein thespherical aberration detection part compares a first focal point shiftamount obtained by detecting changes in the shape of a light spot thatis focused on the light receiving element by applying an astigmatism tolight fluxes created by laser light passing through a region on theinner side of the fourth splitting line, and a second focal point shiftamount obtained by detecting changes in the shape of a light spot thatis focused on the light receiving element by applying an astigmatism tolight fluxes created by laser light passing through the entire region ofthe light splitting device, to generate a spherical aberration errorsignal for detecting the spherical aberration generated between theradiation of the laser light from the light source and the focusing ofthe laser light onto the information recording medium.
 16. The opticalhead according to claim 15, further comprising tilt detection part fordetecting the relative inclination between the objective lens and theinformation recording medium, wherein the interval between the secondand third splitting lines is narrower than the width, in thelongitudinal direction of the information track of the informationrecording medium, of the region in which 0-order light and ±1-orderlight that are diffracted at the information track of the informationrecording medium are superimposed; and wherein the tilt detection partcompares the phase of a first push pull signal obtained by calculatingsignals detected by receiving light fluxes created by laser lightpassing through two regions that are disposed on the outer side of thefourth splitting line and between the second and the third splittinglines, and of a second push pull signal obtained by calculating signalsdetected by receiving light fluxes created by laser light passingthrough the entire region of the light splitting device, to generate atilt error signal for detecting the relative inclination between theobjective lens and the information recording medium.
 17. The opticalhead according to claim 13, wherein the signal indicating the firstfocal point shift amount is SAE2, and the signal indicating the secondfocal point shift amount is SAE 1, and the spherical aberrationcorrection signal SAE is expressed by:SAE=SAE2−k×SAE1, (where k is a constant that substantially satisfiesk=SAE2/SAE1 when there is no spherical aberration and when the focalpoint shift amount is within a predetermined range).
 18. The opticalhead according to claim 13, wherein the light splitting device includesa polarizing hologram.
 19. The optical head according to claim 13,wherein the light receiving element is an integrated lightreceiving/emitting element that is configured as a single unit with thelight source.
 20. The optical head according to claim 13, wherein thelight receiving element is an integrated optical element in which thelight source and the light splitting device are configured as a singleunit.
 21. The optical head according to claim 13, further comprising: aliquid crystal element provided between the objective lens and the lightsplitting device, and spherical aberration corrector for correcting thespherical aberration by changing the phase of wave fronts that passthrough the liquid crystal element due to the application of a voltagein accordance with the spherical aberration correction signal created bythe spherical aberration detection part.
 22. An optical disk device,comprising: an optical head according to claim 17, and a control circuitfor adding an electrical offset to a focus error signal to create apredetermined focal point shift, and determining the constant k suchthat the fluctuations of the spherical aberration correction signalSAE=SAE2−k×SAE1 in a range of the predetermined focal point shift arecontained within a predetermined range.
 23. The optical head accordingto claim 13, further comprising: a collimator lens, provided between theobjective lens and the light splitting device, that converts the laserlight radiated from the light source to substantially parallel light;and spherical aberration corrector for correcting the sphericalaberration by moving the collimator lens in the direction of the opticalaxis of the laser light, in accordance with the spherical aberrationcorrection signal created by the spherical aberration detection part.24. The optical head according to claim 13, wherein the light splittingdevice divides the laser light that passes through the two regions thatare disposed between the second and third splitting lines into at leasta first plurality of ±1-order diffracted light, divides the laser lightthat passes through the two regions on the outer side of the fourthsplitting line into at least a second plurality of ±1-order diffractedlight, and divides the laser light that passes through the four regionsthat are disposed on the same side as the third splitting line to thesecond splitting line into at least a third plurality of ±1-orderdiffracted light; wherein the light receiving element has a firstplurality of light receiving regions, divided into two, that divides thefirst plurality of ±1-order diffracted light into two and receives thatlight, a second plurality of light receiving regions, divided into two,that divides the second plurality of ±1-order diffracted light into twoand receives that light, and a third plurality of light receivingregions that receive each of the third plurality of ±1-order diffractedlight; wherein the first focal point shift amount is obtained based on aplurality of signals detected by the first plurality of light receivingregions, divided into two; and wherein the second focal point shiftamount is obtained based on a plurality of signals detected by thesecond plurality of light receiving regions, divided into two.
 25. Theoptical head according to claim 15, wherein the light splitting devicedivides the laser light that passes through the region on the inner sideof the fourth splitting line into at least 0-order light and a firstplurality of ±1-order diffracted light, divides the laser light thatpasses through the two regions that are disposed on the outer side ofthe fourth splitting line and between the second and the third splittinglines into at least 0-order light and a second plurality of ±1-orderdiffracted light, and divides the laser light that passes through thefour regions that are disposed on the outer side of the second and thirdsplitting lines into at least 0-order light and a third plurality of±1-order diffracted light; wherein the light receiving element has afirst plurality of light receiving regions, divided into four, thatdivides the first plurality of ±1-order diffracted light into four andreceives that light, a second plurality of light receiving regions thatreceive each of the second plurality of ±1-order diffracted light, athird plurality of light receiving regions that receive each of thethird plurality of ±1-order diffracted light, and a fourth lightreceiving regions, divided into four, that divides 0-order light intofour and receives that light; wherein the first focal point shift amountis obtained based on a plurality of signals detected by the firstplurality of light receiving regions, divided into four; and wherein thesecond focal point shift amount is obtained based on a plurality ofsignals detected by the fourth light receiving regions, divided intofour.
 26. The optical head according to claim 14, wherein the lightsplitting device divides the laser light into 0-order light, and aplurality of ±1-order diffracted light; wherein the light receivingelement has a light receiving region, divided into plurality, thatdivides the 0-order light into at least two and receives that light, andwherein the second push pull signal is obtained based on the signaldetected by the light receiving region, divided into plurality.
 27. Theoptical head according to claim 15, wherein the signal indicating thefirst focal point shift amount is SAE2, and the signal indicating thesecond focal point shift amount is SAE1, and the spherical aberrationcorrection signal SAE is expressed by:SAE=SAE2−k×SAE1, (where k is a constant that substantially satisfiesk=SAE2/SAE1 when there is no spherical aberration and when the focalpoint shift amount is within a predetermined range).
 28. The opticalhead according to claim 15, wherein the light splitting device includesa polarizing hologram.
 29. The optical head according to claim 15,wherein the light receiving element is an integrated lightreceiving/emitting element that is configured as a single unit with thelight source.
 30. The optical head according to claim 15, wherein thelight receiving element is an integrated optical element in which thelight source and the light splitting device are configured as a singleunit.
 31. The optical head according to claim 15, further comprising: aliquid crystal element provided between the objective lens and the lightsplitting device, and spherical aberration corrector for correcting thespherical aberration by changing the phase of wave fronts that passthrough the liquid crystal element due to the application of a voltagein accordance with the spherical aberration correction signal created bythe spherical aberration detection part.
 32. The optical head accordingto claim 15, further comprising: a collimator lens, provided between theobjective lens and the light splitting device, that converts the laserlight radiated from the light source to substantially parallel light;and spherical aberration corrector for correcting the sphericalaberration by moving the collimator lens in the direction of the opticalaxis of the laser light, in accordance with the spherical aberrationcorrection signal created by the spherical aberration detection part.